The interplay of various factors with leukemia virus resulting in different forms of lymphosarcoma (leukemia) will be studied using cattle and sheep as models. The various forms of the disease will be induced by inoculation of bovine leukemia virus. The latent period is about two years in experimental sheep and much longer in experimental cattle. These latent periods must be influenced by factors repressing and enhancing development of neoplasia. Emphasis will be placed on the influence of humoral and cell mediated immune systems. These will be correlated with the time of lymphosarcoma. Factors involved in natural transmission and development of different forms of lymphosarcoma will be sought in herds of cattle with a high incidence of the disease.